You're looking at the Pico now?
Well anyways, the D2+ sounds good, great, or else it wouldn't be reccommended. Theres loads of reviews on the D2, almost the same - the LO function. No offense, but i don't think your headphones deserve a Pico, at that price.
D3 python at 200 bucks isnt really good, the D4 sells at 219, with dual dacs, the D3 doesn't have that nor is it rollable. If you're looking to get serious into dac/amps, id really check out the D4. It can run both 9V, 5V, opamp and buffer rollable, dual dac chips, and an amp section compared to the P3+. It's a real stepup from the D2, and you shouldn't be afraid not to notice a difference. Nevertheless, the D2+ is a great choice.
And instead of looking at super expensive portables, i'd get a 150-200 one, and rather upgrade some home components. Even better is to buy some new music. That will probably enhance your listening experiences more than spending 200 bucks extra on a DAC/amp
The D4 is powered at 5V through USB and need batteries included to get the amp section to be driven at 9V. But the amp section will still work via USB only. The D4 does not charge via USB, and must be charged via rechargeable batteries in a AAA charger.
That may sound tough to handle, but dont forget you can rechardge the batteries, while listening to the D4 through USB.